Some water heaters include a secondary heat exchanger for recovering latent heat in addition to a primary heat exchanger for recovering sensitive heat (See Patent Documents 1-4, for example). With this structure, the heat exchange efficiency can be enhanced as compared with the structure designed to recover only sensitive heat from combustion gas. However, when latent heat is recovered from combustion gas, water vapor contained in the combustion gas condenses to form condensate water. Generally, condensate water having absorbed sulfur oxide, nitrogen oxide and so on contained in the combustion gas has a strong acidity of about pH 3.
In the above-described water heater, the secondary heat exchanger may be made of e.g. acid-resistant stainless steel so as not to be corroded due to the strongly acidic condensate water. On the other hand, the primary heat exchanger is generally made of e.g. copper for reducing the manufacturing cost for the entirety of the heat exchangers. Since such primary heat exchanger is not acid-resistant, the primary heat exchanger needs to be so designed that strongly acidic condensate water is not formed accompanying the heat recovery. On the other hand, the secondary heat exchanger is not subjected to such restriction on the design. Therefore, to enhance the heat exchange efficiency of the water heater, it is desired to increase the amount of heat recovery by the secondary heat exchanger as much as possible. Conventionally, therefore, for the secondary heat exchanger, a water tube whose diameter is smaller than that of the primary heat exchanger is often employed as a means to increase the amount of latent heat recovery.
However, the above-described conventional structure has the following drawbacks.
First, when the driving of the water heater is stopped in winter, it is desired to prevent the interior of the water tube of the heat exchanger from freezing. To prevent the freezing without using an antifreeze heater, it is necessary to drain water from the water tube after the water inflow to the water tube is stopped. Such draining of water may be sometimes necessary also at the time of maintenance of the water heater. Since the water tube of the primary heat exchanger has a relatively large diameter, water in the water tube can be smoothly discharged to the outside. In the secondary heat exchanger, however, the diameter of the water tube is relatively small, as noted above. Therefore, due to the surface tension of water generated in the water tube, the water in the water tube may not be smoothly discharged to the outside. As a result, water remains in the water tube and freezes.
Secondly, when condensate water remains adhering to a wide range of the surface of a water tube of the secondary heat exchanger, the heat exchange between the water tube and the combustion gas is hindered by the condensate water, whereby the heat exchange efficiency is deteriorated. Conversely, when much condensate water drops down from a water tube of the secondary heat exchanger, the primary heat exchanger is contaminated and corroded due to the condensate water dropped from the secondary heat exchanger in the case where the primary heat exchanger is arranged directly under the secondary heat exchanger.
The Patent Document 5 discloses a structure in which the water tube of the secondary heat exchanger is inclined so that condensate water formed at a surface of the water tube flows toward the lower end of the water tube and is also evaporated in flowing. However, since the driving of the water heater is not constant, much condensate water may be formed at a surface of the water tube of the secondary heat exchanger. In such a case, with the structure disclosed in the Patent Document 5, part of the condensate water may directly drop from the water tube to contaminate the region below the water tube. Further, all amounts of the condensate water may not be evaporated before reaching the lower end of the water tube. In such a case, unevaporated condensate water may reach the lower end of the water tube to flow below to contaminate the adjacent portions. Moreover, although the water tube is inclined in the water heater of the Patent Document 5, specific measures are not taken to properly drain water from the water tube. Therefore, the draining of water to prevent freezing, for example, cannot be performed easily and properly. Similarly to the Patent Document 5, the Patent Document 6 also discloses a structure in which a water tube of the heat exchanger is inclined to cause condensate water to flow toward the lower end of the water tube of the heat exchanger. Also in this case, it is difficult to properly solve the above-described problems only by such a structure.                Patent Document 1: JP-A 7-167586        Patent Document 2: JP-A 9-159282        Patent Document 3: JP-A 2004-245515        Patent Document 4: JP-A 2004-61069        Patent Document 5: JP-A 2002-39623        Patent Document 6: JP-U 59-55271        